Insurgents posing as police destroyed the golden dome of one of Iraq's holiest Shiite shrines Wednesday, setting off an unprecendented spasm of sectarian violence. Angry crowds thronged the streets, militiamen attacked Sunni mosques, and at least 19 people were killed.

With the gleaming dome of the 1,200-year-old Askariya shrine reduced to rubble, some Shiites lashed out at the United States as partly to blame.

The violence â many of the 90 attacks on Sunni mosques were carried out by Shiite militias â seemed to push Iraq closer to all-out civil war than at any point in the three years since the U.S.-led overthrow of Saddam Hussein.

Many leaders called for calm. "We are facing a major conspiracy that is targeting Iraq's unity," said President Jalal Talabani, a Kurd. "We should all stand hand in hand to prevent the danger of a civil war."

President Bush pledged American help to restore the mosque after the bombing north of Baghdad, which dealt a severe blow to U.S. efforts to keep Iraq from falling deeper into sectarian violence.

"The terrorists in Iraq have again proven that they are enemies of all faiths and of all humanity," Bush said. "The world must stand united against them, and steadfast behind the people of Iraq."

British Prime Minister Tony Blair also condemned the bombing and pledged funds toward the shrine's reconstruction.

U.S. Ambassador Zalmay Khalilzad and the top American commander in Iraq, Gen. George Casey, called the attack a deliberate attempt to foment sectarian strife and warned it was a "critical moment for Iraq."

No one was reported injured in the bombing of the shrine in Samarra.

But at least 19 people, including three Sunni clerics, were killed in the reprisal attacks that followed, mainly in Baghdad and predominantly Shiite provinces to the south, according to the Iraqi Islamic Party, the country's largest Sunni political group.

Many of the attacks appeared to have been carried out by Shiite militias that the United States wants to see disbanded.

In predominantly Shiite Basra, police said militiamen broke into a prison, hauled out 12 inmates, including two Egyptians, two Tunisians, a Libyan, a Saudi and a Turk, and shot them dead in reprisal for the shrine attack.

Major Sunni groups joined in condemning the attack, and a leading Sunni politician, Tariq al-Hashimi, urged clerics and politicians to calm the situation "before it spins out of control."

The country's top Shiite cleric, Grand Ayatollah Ali al-Sistani, sent instructions to his followers forbidding attacks on Sunni mosques, and called for seven days of mourning.

But he hinted, as did Vice President Adil Abdul-Mahdi, that religious militias could be given a bigger security role if the government cannot protecting holy shrines â an ominous sign of the Shiite reaction ahead.

Both Sunnis and the United States fear the rise of such militias, which the disaffected minority views as little more than death squads. American commanders believe they undercut efforts to create a professional Iraqi army and police force â a key step toward the eventual drawdown of U.S. forces.

Some Shiite political leaders already were angry with the United States because it has urged them to form a government in which nonsectarian figures control the army and police. Khalilzad warned this week â in a statement clearly aimed at Shiite hard-liners â that America would not continue to support institutions run by sectarian groups with links to armed militias.

One top Shiite political leader accused Khalilzad of sharing blame for the attack on the shrine in Samarra.

"These statements ... gave green lights to terrorist groups. And, therefore, he shares in part of the responsibility," said Abdul-Aziz al-Hakim, head of the Supreme Council for the Islamic Revolution in Iraq and the former commander of its militia.

The interior minister, who controls the security forces that Sunnis accuse of widepsread abuses, is a member of al-Hakim's party.

The new tensions came as Iraq's various factions have been struggling to assemble a government after the Dec. 15 parliamentary elections.

The Shiite fury sparked by Wednesday's bombings â the third major attack against Shiite targets in as many days â raised the likelihood that Shiite religious parties will reject U.S. demands to curb militias.

The Askariya shrine, also known as the Golden Mosque, contains the tombs of two revered Shiite imams, who are considered by Shiites to be among the successors of the Prophet Muhammad.

No group claimed responsibility for the 6:55 a.m. assault on the shrine in Samarra, a mostly Sunni Arab city 60 miles north of Baghdad, carried out by four insurgents disguised as police. But suspicion fell on Sunni extremist groups.

The top of the dome, which was completed in 1905, collapsed into a crumbly mess, leaving just traces of gold showing through the rubble. Part of the shrine's tiled northern wall also was damaged.

Thousands of demonstrators crowded near the wrecked shrine, and Iraqis picked through the debris, pulling out artifacts and copies of the Muslim holy book, the Quran, which they waved, along with Iraqi flags.

"This criminal act aims at igniting civil strife," said Mahmoud al-Samarie, a 28-year-old builder. "We demand an investigation so that the criminals who did this be punished. If the government fails to do so, then we will take up arms and chase the people behind this attack."

U.S. and Iraqi forces surrounded the Samarra shrine and searched nearby houses. About 500 soldiers were sent to Sunni neighborhoods in Baghdad to prevent clashes.

On Al-Jazeera television, Sunni politician Adnan al-Dulaimi pledged that the violence would not discourage Sunnis from working to form a new government and claimed the Samarra attack was not planned by Sunni insurgents but "a foreign hand aiming to create differences among Iraqis."

National Security Adviser Mouwafak al-Rubaie said 10 people were detained for questioning about the bombing. The Interior Ministry put the number at nine and said they included five guards.

In the hours after the attack, more than 90 Sunni mosques were attacked with automatic rifles and rocket-propelled grenades, burned or taken over by Shiites, the Iraqi Islamic Party said.

Large protests erupted in Shiite parts of Baghdad and in cities throughout the Shiite heartland to the south. In Basra, Shiite militants traded rifle and rocket-propelled grenade fire with guards at the office of the Iraqi Islamic Party. Smoke billowed from the building.

Shiite protesters later set fire to a Sunni shrine containing the seventh century tomb of Talha bin Obeid-Allah, a companion of Muhammad, on the outskirts of Basra.

Protesters in Najaf, Kut and Baghdad's Shiite slum of Sadr City also marched through the streets by the thousands, many shouting anti-American and anti-Israeli slogans and burning those nations' flags.

Tradition says the Askariya shrine, which draws Shiite pilgrims from throughout the Islamic world, is near the place where the last of the 12 Shiite imams, Mohammed al-Mahdi, disappeared. Al-Mahdi was the son and grandson of the two imams buried in the Askariya shrine. Shiites believe he is still alive and will return to restore justice to humanity.

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